This from Bernardo Rossi:

            G-12XL MEMORY BATTERY

If you have the 'low memory battery' alarm, you may not want 
to send it in.  I connected the gps for a couple of days to 
an external power of about 20 Volts, and alarm now has disappeared!

Another user reports that all he did was to turn the unit off 
and plug the unit into 12vdc for about 12 hours.  Warning
gone...